Optical scanners are capable of producing a representation of the image of an original by projecting an image of the original onto optical sensors by means of an optical system. The optical sensors thereby provide signals which represent the image of the original, or a part thereof, as intensity levels. In a colour scanner the image of the original is represented as intensity levels at a given number of colours. The colours, e.g. red, green, and blue, are colour components of a colour represention of the original. Typically the intensity levels are obtained by means of an optical sensor for each of the colour components and the image projected onto the respective optical sensors are filtered by means of respective colour filters. In fact a scanned colour is represented by a spectrum of monochromatic colour components. However, in the following the term colour is used for such a spectrum.
In a practical embodiment the optical sensors are provided as a line detector which comprises a colour line sensor for each colour component. But, due to the fact that each colour line sensor is capable of imaging only one scan line on the original, the original is moved past the line detector at a given feed rate in order to obtain an image of the entire original, that is the original is scanned. Alternatively the line detector may be moved past the original at a given feed rate.
Various types of photosensor devices may be used as colour line sensors, e.g. charge coupled devices (CCD's). A CCD builds up an electrical charge in response to the exposure to light. The magnitude of the electrical charge built up is dependent on the intensity and the duration of the light exposure. The intensity of a picture element (pixel) on the original is imaged by a CCD cell. Multiple CCD cells may be aligned in linear arrays, such that a scan line on the original or a segment of a scan line, comprising multiple pixels, is imaged by means of a CCD array. A line detector may thereby comprise a respective CCD array with a respective colour filter for each colour component in the colour representation. The CCD arrays with the respective colour filters are arranged in parallel and at a mutual distance.
The charge built up in each CCD is sampled and discharged at sampling intervals. Typically the CCD's are sampled at regular sampling intervals, i.e. at a constant sampling rate. Each sample represents the imaged intensity of a pixel for a given colour component.
Since the respective colour line sensors are arranged in parallel at a mutual distance and the image of the original is projected onto the colour line sensors by means of an optical system common for all the colour components, the colour line sensors do not image the same line on the original at a given point of time. That is, the different colour components do not represent the same and one single line, but lines on the original offset a number of scan lines relative to each other.
In order to achieve a representation of a line on the original with all the colour components, the signals from the colour line sensors can be combined electronically.
However, this electronical combination is often rather complex because the scan line offset between the different colour components is changed depending on how the scanner is operated. Often, it is desired to change the resolution of scanning, i.e. the number of pixels per unit area on the original, in order to reduce the amount of data if a coarse representation of the original can be accepted, and vice versa if a fine representation of the original is required, then a larger amount of data is the result. The vertical resolution, perpendicular to the line of scanning, may be changed by adjusting the feed rate, and since the CCD's are sampled at a constant sampling rate, the number of lines scanned is thereby changed, i.e. the number of pixels in a direction which is vertical or transversal to the lines is changed. This change in the vertical resolution of scanning results in a changed scan line offset between the different colour components.
U.S. Pat. No. 5,336,878 discloses a `variable speed single pass color optical scanner`. This scanner is capable of providing a three-colour-representation at a desired line resolution by adjustment of the scanning speed, i.e. the feed rate. The scanner uses three colour line sensors arranged in parallel to detect the colour information and each colour line sensor provides a colour line signal at a sampling rate. According to the actual feed rate, the sampling rate, and the imaged line width on the original the colour line signals are correlated in steps of integer lines.
Thus, the misregistration of colour information may be one-half effective scan line width. Alternatively, the speed selector may be set to allow only certain steps of values of speed choice/scaling, in which case no colour registration error is induced.
However, it is often desired to be able to change the resolution of scanning continuously or at least in fine steps and at the same time avoid this misregistration of colour information.
U.S. Pat. No. 5,642,207 discloses a `color scanner with variable line resolution` which is hereby specifically incorporated by reference in its entirety. This scanner is also capable of providing a three-colour-representation at a desired line resolution, but not by means of changing the feed rate. Instead, the original is scanned at a fixed feed rate, i.e. the number of scanned lines across the original is held fixed. According to a desired resolution, a number of new lines are calculated. These new lines are calculated by means of interpolation between the scanned lines when the number of scanned lines is not an integer multipla of the number of new lines.
This fixed feed rate solution is inexpedient because the scanner is not capable of speeding up the scanning process when a low or coarse resolution is sufficient. Further, when the number of scanned lines is not an integer multipla of the number of new lines, then all colour components of the colour representation are interpolated. This is a frequent situation when an original is scanned because most modern scanners allows for an adjustable resolution that can be continuous--or at least carried out in small steps--which resolution may be adapted specifically for the actual purpose of the scanning.
As stated above are all colour components of the colour representation interpolated, this interpolation will, all other things being equal, introduce an error into the colour representation of the original. If e.g. the common red, green, and blue colour representation is selected, then especially interpolation of the green colour will result in a significant degradation of the perception of sharpness because the green colour is related to the perception of sharpness in the human visual system.
So there exists a problem in the field of optical scanning with variable line resolution, that misregistration is present and that the combination of the contributions from the colour components introduces significant errors.